South Korea's retail sales index fell 0.8 percent on the month in December after increasing 0.9 percent in November. On the year, the index fell 2.2 percent, weakening further from a decline of 0.4 percent previously. Sales of consumer durables rose 0.6 percent on the year after advancing 0.4 percent previously.

Officials also publish a separate report on sales for 25 large retailers. This showed a slowdown in year-over-year growth from 8.7 percent in November to 6.3 percent in December, with another month of strong sales growth for online retailers supported by solid sales for supermarkets and department stores.

Retail Sales measure the total receipts at stores that sell durable and nondurable goods. The data are part of the Monthly Industrial Statistics report.

With consumer spending a large part of the economy, market players continually monitor spending patterns. The pattern in consumer spending is often the foremost influence on stock and bond markets. For stocks, strong economic growth translates to healthy corporate profits and higher stock prices. For bonds, the focus is whether economic growth goes overboard and leads to inflation. Ideally, the economy walks that fine line between strong growth and excessive (inflationary) growth.

Retail sales not only give you a sense of the overall picture, but also the trends among different types of retailers. Especially strong apparel or electronics sales can indicate strength in those industries, for example. These trends from the retail sales data can help you spot specific investment opportunities, without having to wait for a company's quarterly or annual report.